This invention relates generally to gain compensated amplifiers, and more particularly to a gain compensated system that actively compensates the gain of an amplifier due to closed-loop and out-of-loop influences on system gain.
Ideally, power amplifiers provide a constant gain over a wide range of temperatures and frequencies without the need for feedback control. In practice, however, a power amplifier""s gain varies due to changes in temperature, frequency response, and linearity effects. Because amplifiers are non-ideal, gain compression (non-linear behavior) becomes more pronounced the closer an amplifier is driven to saturation. Changes in operating temperature and input frequency also cause an amplifier""s gain to vary. For example, as operating temperature increases, amplifier gain tends to decrease. To overcome these effects, the gain of an amplifier must be actively compensated.
Other anomalies may cause an amplifier""s gain to stray from a desired level. During manufacture of power amplifiers, tolerances among components from unit to unit may cause different amplifiers to have different settling gains under identical operating conditions. Manual calibration of an amplifier is usually required to adjust the amplifier""s gain to specification. In addition, the insertion loss of certain components such as connectors, circulators/isolators, and microstrip lines changes with temperature, which causes slight variations of the total unit gain with temperature. These and other anomalies usually require operator adjustment or intervention during operation of the amplifier, a time consuming and undesired necessity.
Gain variation due to temperature changes can be actively compensated using a closed-loop feedback circuit where the output power is compared to a reference, and any deviation in the output power from the reference causes a control circuit to adjust the power amplifier""s gain. This automatic gain compensation technique works as long as the amplifier""s behavior across a temperature gradient can be determined or predicted. However, such circuits are difficult to mass produce reliably because they often require manual trimming in the manufacturing process to account for closed-loop deviations among components, and so forth, and to account for the out-of-loop temperature effects mentioned above. This trimming or calibration is time-consuming and must be performed unit by unit.
One approach to address some of these effects is to utilize two detectors in an amplifier system to detect an input and output RF signal level and provide these levels to a differential gain control circuit that which is coupled to one or more variable gain amplifier (VGA) stages. The VGA""s compensate for the gain of an entire chain of amplifiers. When the individual amplifier gains vary for any reason (i.e., process, temperature effects, or end of life degradation), the variation in gain causes higher or lower levels of detected output reference signals for a given RF input signal. The gain control circuit drives the VGA up or down as appropriate.
The above approach suffers from several disadvantages, however. For example, the approach may require precise laser trimming during the manufacturing process to calibrate the amplifier to a desired operating gain. In addition, once the amplifier is manufactured, there are no out-of-loop mechanisms for adjusting the gain of the amplifier. Moreover, the above approach provides no mechanism for compensating for out-of-loop influences on system gain.